Sketch-plugin | ImageOptim plugin for the Sketch app | Plugin library

I don't have the full solution but a few good hints:

Coordinates

All layers seem to use a local coordinate system with x and y values between 0 and 1. So you need the layers size and position to calculate the coordinates used in SVG:

x_svg = frame.x + x_layer * frame.width
y_svg = frame.y + y_layer * frame.height

Commands

The mapping of commands is trickier as the Sketch system uses a model different from SVG. The main difference is: SVG uses commands to represent path segments (except for the starting M) while Sketch uses a list of points that are either sharp or soft corners. In order to derive the command, you need to look at two consecutive points. Let's call them p1 and p2:

p1.hasCurveFrom = false and p2.hasCurveTo = false:

That represents a straight line, thus the L command. The coordinate is taken from p2.point.

p1.hasCurveFrom = true and p2.hasCurveTo = true:

That represents a cubic spline, thus the C command. The three coordinates are taken from:

1. p1.curveFrom
2. p2.curveTo
3. p2.point

Further hints

The path starts with an M command to p.point of the first point.

The attribute "isClosed": true obviously indicates a closed path. Therefore, the commands end with Z to close the path.

There are probably many more subtleties that I don't know.

If you want to implement a full Sketch to SVG converter, you will also need to implement the different shape classes and not just triangle, oval and shapePath and you will need to take care of all the other attributes like isFlippedHorizontal, isFlippedVertical, rotation, cornerRadius, resizingType etc.